Method And Equipment For Detecting Failure In Oil Film Formation Of Swash Plate Compressor

ABSTRACT

A swash plate compressor is provided with first conducting means conductive to a shoe of a piston and second conducting means conductive to a swash plate, a voltage with a predetermined intensity is applied from the outside to each of the shoes and the swash plate by a voltage application circuit, and a voltage value of the voltage application circuit is measured by a voltage measuring instrument so that a failure in oil film formation between the shoe and the swash plate can be detected. Thus, there is no need to install precision equipment such as a sensor, a camera or a transmitter inside the compressor body, and the failure in oil film formation can be detected more surely with a simple structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and equipment for detectingfailure in oil film formation of a swash plate compressor used in arefrigerant circuit of an air conditioner for a vehicle, for example.

2. Description of the Related Art

Generally, this type of swash plate compressor is known as comprising aplurality of cylinders provided with an interval to each other in thecircumferential direction in a compressor body, a plurality of pistonsreciprocating in each cylinder, respectively, a swash plate slidablyengaging with one end side of each piston, and a driving shaft forrotating the swash plate, in which the one end side of each piston isslidably brought into contact with the swash plate through a pair ofshoes opposed to each other having the swash plate between them (SeeJapanese Utility Model Laid-Open No. 4-125682, for example).

Also, in the above compressor, an oil film of a lubricant is formedbetween each shoe and the swash plate, but since the shoe and the swashplate mutually slide at a high speed, the lubrication condition isdemanding, and there is a problem that seizing between metal easilyoccurs due to lack of oil film. Then, in order to detect a failure inoil film formation between the shoe and the swash plate, there areproposed a method of measuring a clearance from the shoe by embedding asensor in the swash plate, a method of direct monitoring of a slidingportion using a high-speed camera, a method of external detection byoutputting an optical signal or an electric wave signal when the shoeand the swash plate are brought into contact with each other and thelike.

However, in each of the above detecting methods, since precisionequipment such as a sensor, camera or transmitter is installed insidethe compressor, there are various problems in making it into practice onwhether accurate detection is possible while the compressor isoperating, various operating conditions can be met or detection withoutaffecting performance of the compressor is possible or not.

BRIEF SUMMARY OF THE INVENTION

The present invention was made in view of the above problems, and itsobject is to provide a method and equipment for detecting failure in oilfilm formation of a swash plate compressor which can detect a failure inoil film formation with accuracy in operation of the compressor, respondto various operating conditions and is further capable of detectionwithout affecting performance of the compressor.

In order to achieve the above object, in a method for detecting afailure in oil film formation of a swash plate compressor comprising aplurality of cylinders provided with an interval to each other in thecircumferential direction in a compressor body, a piston reciprocatingin each cylinder, a swash plate slidably engaging with one end side ofthe piston, and a driving shaft for rotating the swash plate, in whichthe one end side of the piston is slidably engaged with the swash platethrough a pair of shoes opposing each other with the swash plate betweenthem, a voltage of a predetermined constant voltage power supply isapplied from outside through a voltage application circuit to conductingmeans for conducting electricity to the shoe of the piston and the swashplate and a voltage value of the voltage application circuit is measuredso as to determine if a value of the measured voltage is equal to orlarger than a predetermined reference value or not.

By this, when a voltage of power supply of the voltage applicationcircuit is applied to the shoe and the swash plate, a clearance by anoil film between the shoe and the swash plate forms electric resistanceand the value of the voltage application circuit becomes smaller thanthe voltage of constant voltage power supply. But if the thickness ofthe oil film becomes smaller, the electric resistance drops and thevoltage value becomes larger. Thus, in the state of lack of oil film orclose to it, the voltage value of the voltage application circuitbecomes substantially equal to the voltage value of the constant voltagepower supply. By this, since it is determined that the value of measuredvoltage becomes equal to or larger than the reference voltage value whenthe voltage value of the voltage application circuit becomes equal to orlarger than a reference voltage value, detection of a failure in oilfilm formation becomes possible based on the determination result.Therefore, detection of a failure in the oil film formation can be madewithout installing precision equipment such as a sensor, camera ortransmitter inside the compressor body.

Also, in order to achieve the above object, in equipment for detectingfailure in oil film formation of a swash plate compressor comprising aplurality of cylinders provided with an interval to each other in thecircumferential direction in a compressor body, a piston reciprocatingin each cylinder, a swash plate slidably engaging with one end side ofthe piston, and a driving shaft for rotating the swash plate, in whichthe one end side of the piston is slidably engaged with the swash platethrough a pair of shoes opposing each other with the swash plate betweenthem, there are provided first conducting means for conductingelectricity to the shoe of the piston, second conducting means forconducting electricity to the swash plate, a voltage application circuitconnected from the outside to the first and the second conducting meansfor applying a voltage of a predetermined intensity to the first and thesecond conducting means, a voltage measuring instrument for measuring avoltage value of the voltage application circuit, and a determiningprocessor for determining if the value of the measured voltage of thevoltage measuring instrument is equal to or larger than a predeterminedreference value.

By this, when a voltage of power supply of the voltage applicationcircuit is applied to the shoe and the swash plate through the first andthe second conducting means, a clearance by the oil film between theshoe and the swash plate forms electric resistance and the value of thevoltage application circuit becomes smaller than the voltage of constantvoltage power supply, but if the thickness of the oil film becomessmaller, the electric resistance drops and the voltage value increases.Thus, in the state of lack of oil film or close to it, the voltage valueof the voltage application circuit becomes substantially equal to thevoltage value of the constant voltage power supply. By this, when thevoltage value of the voltage application circuit becomes equal to orlarger than the reference voltage value, it is determined by thedetermining processor that the value of the measured voltage of thevoltage measuring instrument becomes equal to or larger than thereference voltage value, and detection of a failure in oil filmformation can be made possible based on the determination result.Therefore, detection of a failure in the oil film formation can be madewithout installing precision equipment such as a sensor, camera ortransmitter inside the compressor body.

According to the present invention, since a failure in the oil filmformation can be detected without installing precision equipment such asa sensor, camera or transmitter inside the compressor body, the failurein oil film formation can be detected more surely with a simplestructure. Therefore, the failure in the oil film formation can bedetected with accuracy in operation of the compressor, diversifiedoperating conditions can be responded, and moreover, the failure in oilfilm formation can be detected without affecting performance of thecompressor, which is extremely advantageous in practical application.

The above object as well as the other objects, features and advantagesof the present invention will be made clear in the following descriptionand the attached drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is an outline side view of equipment for detecting failure in oilfilm formation showing an embodiment of the present invention;

FIG. 2 is a sectional view of an essential part of the equipment fordetecting failure in oil film formation;

FIG. 3 is an enlarged sectional view of the essential part of theequipment for detecting failure in oil film formation;

FIG. 4 is a side view of a piston;

FIG. 5 is a partially exploded side view of the piston;

FIG. 6 is a graph showing a relation between a voltage and a clearance;and

FIG. 7 is a partially exploded side view showing another embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 to 6 show an embodiment of the present invention. A swash platecompressor shown in them comprises a compressor body 10 suctioning anddischarging a refrigerant, a plurality of pistons 20 provided inside thecompressor body 10, a swash plate 30 slidably engaged with one end sideof each piston 20, and a driving shaft 40 for rotating the swash plate30.

The compressor body 10 comprises a housing 11, a cylinder block 12, acylinder head 13 and a valve plate 14, which are coupled with each otherin the axial direction by a bolt, not shown.

The housing 11 is formed in the cylindrical state with one end sideblocked, while the other end side is opened on the cylinder block 12side.

The cylinder block 12 has a plurality of cylinders 12 a extending in theaxial direction of the compressor body 10, and both ends of eachcylinder 12 a are opened on the housing 11 side and the valve plate 14side, respectively.

The cylinder head 13 has a refrigerant discharge chamber 13 a and arefrigerant suction chamber 13 b, and the refrigerant discharge chamber13 a and the refrigerant suction chamber 13 b are opened on the cylinderblock 12 side, respectively. The refrigerant discharge chamber 13 a isprovided at a center portion of the cylinder head 13 and communicateswith a discharge port (not shown) to the outside. The refrigerantsuction chamber 13 b is provided in the periphery of the refrigerantdischarge chamber 13 a and communicates with a suction port (not shown)from the outside.

The valve plate 14 is arranged between the cylinder block 12 and thecylinder head 13 and has a plurality of refrigerant discharge holes 14 aand refrigerant suction holes 14 b communicating with each cylinder 12a. On one face side of the valve plate 14 (on the side of the cylinderhead 13), a discharge-side plate 14 c is arranged, and on thedischarge-side plate 14 c, a discharge valve (not shown) foropening/closing each refrigerant discharge hole 14 a is provided. Also,on the other face side of the valve plate 14 (on the side of thecylinder block 12), a suction-side plate 14 d is arranged, and on thesuction-side plate 14 d, a suction valve (not shown) for opening/closingeach refrigerant suction hole 14 b is provided.

Each piston 20 is provided slidably in each cylinder 12 a with anengagement portion 21 to be engaged with the swash plate 30 formed atits one end. The engagement portion 21 has a pair of semispherical facesopposed to each other, and a semispherical shoe 22 is slidably providedbetween each semispherical face.

The swash plate 30 is provided capable of inclination with respect tothe axial direction of the driving shaft 40, and with its peripheraledge portion, the engagement portion 21 of each piston 20 is engagedslidably through the shoe 22. The swash plate 30 is coupled capable ofinclination to a rotating plate 31 rotating integrally with the drivingshaft 40 so as to rotate around the driving shaft 40 by the rotatingplate 31 and to reciprocate each piston 20 by a stroke amountcorresponding to its inclination angle. Also, between each shoe 22 andthe swash plat 30 is formed an oil film by a lubricant circuiting insidethe compressor body 10. In this case, anon-compressed surface (thesurface not receiving a reaction force of the piston 20) of the swashplate 30 is covered by a thin-film insulating member 30 a so that thenon-compressed surface of the swash plate 30 and one of the shoes 22 areelectrically insulated by the insulating member 30 a. By this,electricity is conducted only through the compressed surface (thesurface receiving the reaction force of the piston 20) of the swashplate 30 and the other shoe 22. Instead of the insulating member 30 a,the non-compressed surface of the swash plate 30 may be given surfaceprocessing made of insulating coating.

The driving shaft 40 is rotatably supported by the housing 11 at its oneend side through a bearing 41, while the other end side is rotatablysupported by the cylinder block 12 through a bearing 42.

Next, the equipment for detecting a failure in oil film formation ofthis embodiment will be described. This equipment for detecting failurein oil film formation 50 comprises a terminal 51 provided on the outerface side of the housing 11, a conductive member 52 provided on the sideof the piston 20, a contact pin 53 as a contact member provided at thepiston 20, a voltage application circuit 54 for applying a voltage of apredetermined intensity to each shoe 22 and the swash plate 30, avoltage measuring instrument 55 for measuring a voltage value of thevoltage application circuit 54, and a determining processor 56 fordetermining if the measured voltage of the voltage measuring instrument55 is equal to or larger than a predetermined reference value or not,and the terminal 51, the conductive member 52 and the contact pin 53 areprovided on both sides in the width direction of the piston 20,respectively.

The terminal 51 has a screw hole 51 a for bolt screwing, and aconductor, not shown, of the voltage application circuit 54 isconnected.

The conductive member 52 is made of a metal member extending in thesliding direction of the piston 20 and fastened to the housing 11 by abolt 52 a screwed to the terminal 51. In this case, the bolt 52 a isscrewed to the terminal 51 by insertion through a through hole 11 aprovided at the housing 11, and a sleeve 52 b which insulates the bolt52 a and the housing 11 is inserted through the through hole 11 a. Also,an insulating film 52 c is formed on the inner circumferential face ofthe housing 11, and the conductive member 52 and the housing 11 areinsulated by the insulating film 52 c.

The contact pin 53 is made of conductive metal and slidably insertedinto a hole 20 a provided on the side face of the piston 20. In thiscase, a spring 53 a is provided within the hole 20 a so that the contactpin 53 is pressed into contact with the side face of the conductivemember 52 by the spring 53 a.

Also, at the piston 20, a conducting spring 57 as an elastic memberconductive with the shoe 22 is provided, and the conducting spring 57 isfixed to the piston 20 so that its one end contacts the other shoe 22.

The voltage application circuit 54 comprises a constant voltage powersupply 54 a and a variable resistor 54 b connected in series, and theconstant voltage power supply 54 a generates a direct current voltagewith a predetermined intensity (2.0 [V], for example). The voltageapplication circuit 54 is connected to the terminal 51 at one end side(the variable resistor 54 b side) with the other end side (the powersupply 54 a side) to the housing 11 of the compressor body 10.

The voltage measuring instrument 55 is connected to the voltageapplication circuit 54 in parallel with the variable resistor 54 b so asto measure the voltage of the variable resistor 54 b.

The determining processor 56 is connected to the voltage measuringinstrument 55 so as to determine whether the voltage value measured bythe voltage measuring instrument 55 is equal to or larger than apredetermined reference voltage value V (1.9 [V], for example) or not.

In the equipment for detecting a failure in oil film formation 50, theterminal 51, the conductive member 52, the contact pin 53, the piston 20and the shoe 22 are conducted sequentially as first conducting means,and the housing 11 of the compressor body 10, the driving shaft 40 andthe swash plate 30 are conducted sequentially as second conductingmeans. In this case, the piston 20 and the shoe 22 are also conducted bythe conducting spring 57.

In the compressor constructed as above, when the driving shaft 40 isrotated by a driving force from the outside, each piston 20 reciprocateswithin each cylinder 12 a according to the inclination angle of theswash plate 30. By this, the refrigerant of the refrigerant suctionchamber 13 b is sucked into each cylinder 12 a and discharged into therefrigerant discharge chamber 13 a. At that time, a pressure inside thehousing 11 is regulated by pressure control means, not shown, and by adifferential pressure generated between the refrigerant suction chamber13 b and the inside of the housing 11, the inclination angle of theswash plate 30 is changed according to the pressure applied on the otherend side (the side of the housing 11) of each piston 20. By this, astroke amount of each piston 20 is increased or decreased, and thedischarge amount of the refrigerant is changed.

Also, in the equipment for detecting failure in oil film formation 50 ofthis embodiment, the voltage of the power supply 54 a of the voltageapplication circuit 54 is applied to each shoe 22 and the swash plate30, and the voltage of the variable resistor 54 b is measured by thevoltage measuring instrument 55. At that time, since a clearance isformed by an oil film of lubricant between the shoe 22 and the swashplate 30, this clearance becomes electric resistance and the value ofthe measured voltage becomes smaller than the voltage of the powersupply 54 a. That is, as shown in FIG. 5, if the thickness of the oilfilm is equal to or larger than a normal value (6 [μm], for example), itbecomes substantially 0 [V], but if the thickness of the oil filmbecomes smaller, the voltage value becomes larger and in the case oflack of the oil film or in the state close to it, the voltage valuebecomes substantially the voltage value of the power supply 54 a (2 [V],for example). At that time, whether the value of the measured voltage islarger than the reference voltage value V (1.9 [V], for example) or notis determined by the determining processor 56, and a failure in oil filmformation is detected based on the determination result. That is, if thevalue of the measured voltage is determined as being equal to or largerthan the reference voltage value V by the determining processor 56,predetermined abnormality processing actions including stop of drivingof the compressor or actuation of an alarm device, not shown, are taken.

In this way, according to this embodiment, the voltage of apredetermined intensity is applied by the voltage application circuit 54to each shoe 22 of the piston 20 and the swash plate 30 from the outsideand the voltage value of the voltage application circuit 54 is measuredby the voltage measuring instrument 55. And since the failure information of an oil film between the shoe 22 and the swash plate 30 isdetected by determining whether the value of the measured voltage isequal to or larger than the predetermined reference value or not by thedetermining processor 56, there is no need to install precisionequipment such as a sensor, camera or transmitter inside the compressorbody 10, and the failure in oil film formation can be detected moresurely with a simple structure. Therefore, the failure in oil filmformation can be detected with accuracy in operation of the compressor,diversified operating conditions can be responded, and moreover, thefailure in oil film formation can be detected without affectingperformance of the compressor, which is extremely advantageous inpractical realization.

In this case, since a failure in oil film formation can be easilydetected by connecting the voltage application circuit 54 to thecompressor from the outside, not only that it can be provided in an airconditioning device for a vehicle for detecting a failure in oil filmformation of a compressor mounted on a vehicle but even though it is notprovided in the air conditioning device for a vehicle, the failure inoil film formation can be easily detected by connecting the voltageapplication circuit 54 to the compressor at shipment from a plant ormaintenance, which can improve versatility.

Also, since it is so constructed that the terminal 51, the conductivemember 52, the contact pin 53, the piston 20 and the shoe 22 aresequentially conducted by providing the terminal 51 to which the one endside of the voltage application circuit 54 can be connected on the outersurface side of the compressor body 10, providing the conductive member52 conductive to the terminal 51 on the side of the piston 20, and byproviding the conductive contact pin 53 slidably contacting theconductive member 52 at the piston 20, the one end side of the voltageapplication circuit 54 can be made conductive to the shoe 22 only byadding a simple part such as the terminal 51, the conductive member 52and the contact pin 53, which is extremely advantageous for costreduction.

Moreover, since the compressor body 10, the driving shaft 40 and theswash plate 30 are sequentially conducted, the other end side of thevoltage application circuit 54 can be conducted to the swash plate 30only by connecting the other end side of the voltage application circuit54 to the compressor body 10 without adding an exclusive conductivepart, which is extremely advantageous for cost reduction.

Also, since each contact pin 53 is pressed into contact with theconductive member 52 by the spring 53 a, the contact pin 53 can besurely brought into contact with the conductive member 52, andconducting between the conductive member 52 and the contact pin 53 canbe made surely.

In this case, since the conductive member 52 and the contact pin 53 areprovided on both sides in the width direction of the piston 20,respectively, even if the piston 20 is displaced in the width directionduring a reciprocating motion, one of the contact pins 53 can be broughtinto contact with either one of the conductive members 52 all the time,and conducting between the conductive member 52 and the contact pin 53can be made more surely. Moreover, since the circumferential rotation ofthe piston 20 can be regulated by each conductive member 52 arranged onboth sides in the width direction, the piston 20 can be reciprocatedstably all the time without separately providing a part for preventingrotation.

Also, since the conducting spring 57 with the one end in contact withthe other shoe 22 is provided at the piston 20 so that the piston 20 andthe other shoe 22 are also conducted by the conducting spring 57, theconducting between the piston 20 and the shoe 22 can be made moresurely, and drop of conductivity caused by the oil film between thepiston 20 and the other shoe 22 can be surely prevented.

Moreover, since the voltage value of the variable resistor 54 b providedat the voltage application circuit 54 is measured by the voltagemeasuring instrument 55, the correlation between the voltage and theclearance between the swash plate/shoe shown in FIG. 6 can bearbitrarily set by regulating the resistance value of the variableresistor 54 b.

Also, since the one face (non-compressed surface) of the swash plate 30is insulated from the one shoe 22 by the insulating member 30 a, onlybetween the other face (compressed surface) of the swash plate 30 andthe other shoe 22 can be conducted, and a failure in oil film formationon the compressed surface side of the swash plate 30 where lubricationstate is demanding can be surely detected.

The conductive member 52 and the contact pin 53 of the equipment fordetecting failure in oil film formation in the above embodiment may beprovided at each piston 20, but they may be provided only some of thepistons 20.

Also, in the above embodiment, an example that the one face(non-compressed surface) of the swash plate 30 is insulated from the oneshoe 22 is shown, but as shown in FIG. 7, if the other face (compressedsurface) of the swash plate 30 is insulated from the other shoe 22 by aninsulating member 30 b, the failure in oil film formation on thenon-compressed surface of the swash plate 30 can be detected.

The example described in this specification is only for an example andnot limiting. The scope of the invention is indicated in the appendedclaims and all the variations included in the meaning of the claims areincluded in the present invention.

1. A method for detecting failure in oil film formation of a swash platecompressor comprising a plurality of cylinders provided with an intervalto each other in the circumferential direction in a compressor body, apiston reciprocating in each cylinder, a swash plate slidably engagingwith one end side of the piston, and a driving shaft for rotating theswash plate, in which the one end side of the piston is slidably engagedwith the swash plate through a pair of shoes opposing each other withthe swash plate between them, wherein a voltage of a predeterminedconstant voltage power supply is applied through a voltage applicationcircuit to conducting means for conducting electricity to the shoe ofthe piston and the swash plate; and a voltage value of the voltageapplication circuit is measured so as to determine if a value of themeasured voltage is equal to or larger than a predetermined referencevalue or not.
 2. Equipment for detecting failure in oil film formationof a swash plate compressor comprising a plurality of cylinders providedwith an interval to each other in the circumferential direction in acompressor body, a piston reciprocating in each cylinder, a swash plateslidably engaging with one end side of the piston, and a driving shaftfor rotating the swash plate, in which the one end side of the piston isslidably engaged with the swash plate through a pair of shoes opposingeach other with the swash plate between them, comprising: firstconducting means for conducting electricity to the shoe of the piston;second conducting means for conducting electricity to the swash plate; avoltage application circuit connected from the outside to the first andthe second conducting means for applying a voltage of a predeterminedconstant voltage power supply to the first and the second conductingmeans; a voltage measuring instrument for measuring a voltage value ofthe voltage application circuit; and a determining processor fordetermining if the value of the measured voltage of the voltagemeasuring instrument is equal to or larger than a predeterminedreference value.
 3. The equipment for detecting failure in oil filmformation of a swash plate compressor according to claim 2, furthercomprising: a terminal provided on the outer face side of the compressorbody and to which the voltage application circuit can be connected; aconductive member provided on the side of the piston for conductingelectricity to the terminal; and a conductive contact member provided atthe piston and slidably contacting the conductive member, wherein thefirst conducting means is formed so that the terminal, the conductivemember, the contact member, the piston and the shoe are sequentiallyconducted.
 4. The equipment for detecting failure in oil film formationof a swash plate compressor according to claim 2, wherein the secondconducting means is formed so that the compressor body, the drivingshaft and the swash plate are sequentially conducted.
 5. The equipmentfor detecting failure in oil film formation of a swash plate compressoraccording to claim 2, wherein the contact member is provided so that itis pressed into contact with the conductive member by a spring.
 6. Theequipment for detecting failure in oil film formation of a swash platecompressor according to claim 2, wherein the conductive member and thecontact member are provided on both sides in the width direction of thepiston, respectively.
 7. The equipment for detecting failure in oil filmformation of a swash plate compressor according to claim 6, wherein eachof the conductive members is provided so as to regulate rotation of thepiston in the circumferential direction.
 8. The equipment for detectingfailure in oil film formation of a swash plate compressor according toclaim 2, wherein a conductive elastic member having one end side fixedto the piston so that the other end is in contact with the shoe isprovided at the piston.
 9. The equipment for detecting failure in oilfilm formation of a swash plate compressor according to claim 2, whereinthe voltage measuring instrument measures a voltage value of a variableresistor provided at the voltage application circuit.
 10. The equipmentfor detecting failure in oil film formation of a swash plate compressoraccording to claim 2, wherein the swash plate is insulated from one ofthe shoes by an insulating member.
 11. Equipment for detecting failurein oil film formation of a swash plate compressor comprising a pluralityof cylinders provided with an interval to each other in thecircumferential direction in a compressor body, a piston reciprocatingin each cylinder, a swash plate slidably engaging with one end side ofthe piston, and a driving shaft for rotating the swash plate, in whichthe one end side of the piston is slidably engaged with the swash platethrough a pair of shoes opposing each other with the swash plate betweenthem, comprising: first conducting means for conducting electricity tothe shoe of the piston; second conducting means for conductingelectricity to the swash plate; a voltage application circuit connectedfrom the outside to the first and the second conducting means forapplying a voltage of a predetermined constant voltage power supply tothe first and the second conducting means; a voltage measuringinstrument for measuring a voltage value of the voltage applicationcircuit; a determining processor for determining if the value of themeasured voltage of the voltage measuring instrument is equal to orlarger than a predetermined reference value; a terminal provided on theouter face side of the compressor body and to which the voltageapplication circuit can be connected; a conductive member provided onthe side of the piston for conducting electricity to the terminal; and aconductive contact member provided at the piston and slidably contactingthe conductive member, wherein the first conducting means is formed sothat the terminal, the conductive member, the contact member, the pistonand the shoe are sequentially conducted; the second conducting means isformed so that the compressor body, the driving shaft and the swashplate are sequentially conducted; the contact member is provided so thatit is pressed into contact with the conductive member by a spring; theconductive member and the contact member are provided on both sides inthe width direction of the piston, respectively; each of the conductivemembers is provided so as to regulate rotation of the piston in thecircumferential direction; a conductive elastic member having the otherend side fixed to the piston so that one end is in contact with the shoeis provided at the piston; the voltage measuring instrument isconstructed to measure a voltage value of a variable resistor providedat the voltage application circuit; and the swash plate is insulatedfrom one of the shoes by an insulating member.